The electric power grid systems in developed countries have a growing need for energy storage, especially as wind and solar power provide an increasing portion of energy requirements. To date, pumped hydro systems have provided most of the world's electric energy storage, but the best of these resources have been tapped and environmental concerns make new development difficult. Indeed, the electric energy storage capacity of most countries is less than 5% of their grid power capacity. Modern electric grid systems could operate more efficiently than present, if energy storage could be increased to 10-20% of grid power capacity. As the world moves toward renewable energy generation (e.g., wind and solar), the need for energy storage may eventually grow to over 30% of the grid power rating and may have to be sustainable for 8-12 hours.
Pumped hydro energy systems are a well known and widely used method of energy storage. They are similar to a hydroelectric generating plant, in that they typically include a controlled release reservoir and water turbines driving electric generators. The pumped hydro systems, however, go beyond a hydroelectric facility in that they provide a reservoir at the lower level and an electric motor-driven pump that can pump the water from the lower reservoir to the upper reservoir during times when the power companies have excess generating capacity, typically at night and on weekends. Newer pumped hydro systems use a combination pump/turbine-motor/generator that combines a reversible water turbine or pump and motor/generator into a single unit. The Frances type water turbine is typically used and is available in sizes from under 10 megawatts to over 200 megawatts. These units are extremely efficient, with round trip energy efficiencies that can exceed 80%.
A relatively new concept for energy storage is the compressed air system. These systems use large underground cavities or caverns to store compressed air. A motor-driven compressor at the ground surface compresses the air during times of excess generating capacity, and a gas turbine generator is then used to recover the energy, when needed. The compressed air cannot be used to simply drive an air motor generator due to the extreme cooling of the air as it expands through the turbine. Accordingly, known compressed air energy storage systems require the use of gas turbine driven generators to heat the compressed air and recover the energy. Such systems recover the energy of the compressed air primarily by increasing the power and efficiency of gas turbine generators. With normal gas turbines, nearly two-thirds of the energy is used to compress the air that is needed to burn the gas. With compressed air energy systems, the round trip energy efficiency can approach 80%, but the economies of this action depend upon the cost of the natural gas required to recover the energy.
It is particularly noteworthy that pumped hydro energy storage has advantages over compressed air systems, including the ability to start up quickly, provide “spinning reserves,” and provide voltage and frequency regulation to stabilize the associated power grid.
While the prior art has employed both pumped hydro and compressed air energy storage systems with some success, it appears that a combined system, capitalizing on the benefits of each, has not been envisioned. Accordingly, there remains a need in the art for a compressed air pumped hydro energy system, utilizing both pumped water and compressed air to effect power generation and energy storage for application to the power grid of cities or other specified regions.